The present invention relates to a method for hydrometallurgically processing lead materials. In particular, it relates to the use of fluoro compounds to leach the lead materials so they can be further processed. More particularly, it relates to fluorotitanate compounds and their use in hydrometallurgically processing lead materials.
It is known to produce lead metal from galena by smelting. The problem with this method is that it creates a considerable amount of lead (and other metal) dust and sulfur dioxide gas, which presents a substantial environmental problem.
In response to this and other problems, hydrometallurgical processes have been proposed. In one process, the galena is leached in an aqueous solution of ferric chloride with sodium chloride. The sulfur-containing residue is filtered and the lead chloride is electrolyzed as a fused salt. The problem with this method is that is complex, not environmentally safe, consumes a large amount of energy and does not produce lead of sufficiently high purity.
In another process, described in U.S. Pat. No. 5,039,337, the galena is leached with an acidic aqueous solution of ferric fluoborate to form ferrous fluoborate, lead fluoborate and elemental sulfur. The solution of ferrous fluoborate and lead fluoborate is sent to a diaphragm electrolytic cell, where pure lead is deposited at the cathode and the ferrous ion is oxidized to ferric ion at the anode. As a result, the ferric fluoborate is regenerated so that it can be reused for leaching the galena.
In a similar process, described in U.S. Pat. No. 5,441,609, the source of lead is leached with ferric fluoborate in fluoroboric acid to cause the lead to be dissolved in solution. The solution containing the lead is fed to an electrolytic cell having a diaphragm so that the lead is deposited in pure form at the cathode and the ferrous ions are oxidized to ferric ions at the anode to generate a solution of ferric fluoborate. The ferric fluoborate can then be recycled to leach more lead.
While these two processes produce lead of sufficient quality, one particularly important disadvantage is that fluoroboric acid is very expensive and the solution recycle is cumbersome. For example, most environmental regulations require, for solution disposal purposes, that boron be less than 1 ppm. Unfortunately, the boron present in an aqueous solution as a borate compound is extremely stable and highly soluble over a wide range of pH. Thus, when fluoborate is used, a complex waste solution recycle and disposal scheme must be in place. Because of the complexity of the recycle system and the cost of fluoroboric acid (boron) as well as the cost of the necessary recycle equipment, there is a need for a process to economically and effectively produce suitable high purity lead.
The process according to the present invention solves that need by using a leachate, fluotitanate, that is less expensive than the fluoborate. Using current day commodity prices, the cost of one volume of fluotitanate leach solution is less than 40% the corresponding cost for the fluoborate system.
The present invention provides a process for hydrometallurgical processing of lead. In general, the process is directed to the use of fluotitanate in the processing of lead.
In one embodiment, the process comprises the following steps. Leaching a source of lead with a leach solution containing fluoro compounds to dissolve the lead and to form a leachate solution containing ferrous fluro compounds, lead fluro compounds and elemental sulfur. In particular, the leach solution is an aqueous solution containing one or more fluoro compounds selected from the group consisting of fluosilicate, fluotitanate, fluoborate, and mixtures thereof. Preferably, when fluoborate is used as an ingredient of the leach solution, it is used in combination with other fluoro compounds such as fluosilicate and fluotitanate in order to reduce the cost of the leach solution. In this regard, fluoborate is generally used in amounts no greater than 50% by weight of the leach solution. In a preferred embodiment, the leach solution contains a major amount of ferric fluotitanate. In this preferred embodiment, the lead-containing material is leached to form a solution containing ferrous fluotitanate, lead fluotitanate, and elemental sulfur.
The resulting leachate solution is preferably filtered to remove any residue and to form a lead-enriched solution.
Thereafter, according to one embodiment, the lead-enriched solution is fed to an electrolytic cell having a diaphragm, in which lead is deposited in pure form at the cathode and ferrous ions are oxidized to ferric ions at the anode. Preferably, the resulting ferric compounds are recycled to form at least a portion of the leach solution.
According to another embodiment of the invention, the lead-enriched solution is passed into an electrowinning cell wherein the solution is subjected to an electrowinning reaction. The electrowinning of the lead fluoro compounds will result in the production of metallic lead in the form of sheets on the cathodes. The ferric compounds formed during the electrolysis reaction may then be recycled to form at least a portion of the leach solution.
According to yet another embodiment of the present invention, fluotitanate is used as the electrolyte in the electrorefining of lead. In this embodiment, impure lead is placed as an anode or anodes in an electrolyzing solution containing fluotitanate and lead is deposited on the cathode or cathodes. In general, the electrorefining can be accomplished according to the Betts process, details of which are disclosed in U.S. Pat. Nos. 679,824, 713,277 and 713,278, the relevant portions of which are incorporated herein by reference. Alternatively, it is believed that the electrorefining can be accomplished using the bipolar process described in U.S. Pat. No. 4,177,117 and 4,416,746, the relevant portions of which are incorporated herein by reference.